Optical screens typically use cathode ray tubes (CRTs) for projecting images onto the screen. The standard screen has a width to height ratio of 4:3 with 525 vertical lines of resolution. An electron beam is scanned both horizontally and vertically across the screen to form a number of pixels, which collectively form the image.
Conventional cathode ray tubes have a practical limit in size, and are relatively deep to accommodate the required electron gun. Larger screens are available which typically include various forms of image projection. However, such screens have various viewing shortcomings including limited viewing angle, resolution, brightness, and contrast, and such screens are typically relatively cumbersome in weight and shape. Furthermore, it is desirable for screens of any size to appear black in order to improve viewing contrast. However, it is impossible for direct view CRTs to actually be black because they utilize phosphors to form images, and those phosphors are non-black.
Optical panels used for viewing images may be made by stacking waveguides. Such a panel may be thin in its depth compared to its height and width, and the cladding of the waveguides may be made black to increase the black surface area. It is known in the art that waveguide component are utilized for transmission of light. It is further known in the art that a waveguide has a central transparent core that is clad with a second material of a lower refractive index. In order to provide total internal reflection of light within this waveguide, the central core has a higher refractive index of refraction than the clad. By adjusting the difference in refractive index the acceptance angle of incoming light may be varied. The larger the difference in refractive index, the larger the incoming light acceptance angle.
However, optical waveguides of the step index cladding type that are stacked and fused together have some significant drawbacks. In the formation of a large optical panel using stepped index clad waveguides many layers are stacked on top of each other and adhered to each other. In a typical 50″ diagonal screen there may be several hundreds or even thousands of waveguides that are adhered to one another. Handling and cutting many strips of thin polymer is very difficult. The compatibility of materials that have a refractive index difference from core to clad is limited. This may contribute to problems such as inadequate adhesion between layers. Such incompatibility may result in layer to layer interface problems such as air gaps, rough surfaces, or layer separation. These types of problems may cause a loss of light at each bounce at the interface between the core layer and surrounding cladding layers. Furthermore when stacked, fused and saw cut, panels that are made from large blocks, may have an additional problem with the light inlet and light viewing side of the panel. These surfaces typically are very rough and non-uniform and they interfere with the light entering and leaving the waveguide. Typically the rough non-uniform surface result in light scattering and haze problems that reduce the viewing clarity of the image. Therefore, the amount of light loss that occurs in optical panels becomes a significant detriment to the overall efficiency and performance of the optical panel, as well as the quality, such as brightness and sharpness, of the image. It is desirable to find a means to improve the viewing clarity, sharpness and brightness of the image.
There are a limited number of materials that can be used in combination between the core and clad that provide the desired delta refractive index, that provide adequate adhesion between the layers and are capable of absorbing ambient room light, and are also light in weight. These limitations have resulted in the need to use plastic polymer in place of glass. While stacking and fusing together many layers of plastic is relatively easy, there are many problems when individual panels are cut. Both the inlet and viewing surface are rough and need to be ground and polished in order to provide a surface that does not interfere with the luminous flux of light. Grinding and polishing plastics is difficult to control. It is important to have a means of controlling or modifying the inlet and viewing surface smoothness of the cut panels to assure that the optical characteristics are optimized. In stepped refractive index clad waveguides of the type described in U.S. Pat. Nos. 6,002,826, 6,301,417 and 5,625,736 it is important to control the surface smoothness of the optical panel. In these patents a means of polishing with diamonds is discussed. There remains a need for improved control of surface smoothness that is less labor intensive and time consuming.
It is a problem to be solved to provide improved waveguides that have a smooth inlet surface and viewing surface that can be obtained without excessive grinding and polishing.